1. Field of the Invention
This invention concerns pressure sensors and in particular their application to a pressure monitoring device in an infusion apparatus.
2. Description of Prior Art
Infusion pumps are used to inject medicamentous substances into a patient's body, usually intravascularly, but equally by other appropriate access channels.
It is useful to be able to monitor the flow pressure of the fluid as it comes down from the infusion pump, so as to detect any possible anomaly in the apparatus in operation. Indeed, various incidents may occur: the pump may race or adversely stop working, the catheter through which the medicamentous substance is injected may progressively close up with blood deposits, the infusion tube may be plicated, a leak may occur at any point in the infusion tube, etc . . .
Switches are currently used to monitor flow pressure in an infusion tube, but they work as a two-way system (on or off) and are activated by a substantial variation in pressure in relation to the nominal operating pressure. Such devices are not very precise, with measurement precision being of the order of 50% of the value of measured pressure. They are often used simply to control the shutting down of the motor driving the pump when the pressure exceeds a certain level.
Other more sophisticated devices are also known, such as those described in patents U.S. Pat. No. 4,446,344 and U.S. Pat. No. 4,431,425. The devices described in said patents comprise switches capable of not only stopping the motor when the pressure is too high but also restarting the motor once the pressure has descended to an acceptable value. However, there still remains the disadvantage of insufficient precision.
A device specially designed for the medical field is described in patent DE-A-2,461,424. Said device consists of a chamber filled with glycerine which transmits pressure to a sensor, said pressure being exerted on the wall of the sensor's chamber. Such a device with liquid (glycerine) as a pressure transmission fluid is difficult to use for measuring pressure in an infusion tube. Indeed, so that there may be an instantaneous reaction to variations of pressure in an infusion tube, it is necessary for the transmission fluid response to be linear, and such is not the case of liquid.